Chapter 14 Metabolic Diversity of organisms

What does the oxidation state indicate in metabolism?

The degree of electron loss or gain.

• More positive → oxidized (electron-poor) → good electron acceptor

• More negative → reduced (electron-rich) → good electron donor

better electron donor: CH₄ (C = -4) → highly reduced → can donate electrons

• CO₂ (C = +4) → highly oxidized → accepts electrons

What determines ATP yield in respiration?

Difference in redox potential (ΔE₀’) between donor and acceptor.

• Bigger difference → more energy → more ATP

Define respiration in microbes.

Electron transfer from donor → acceptor via ETC, generating ATP via PMF.

What are the two components of the proton motive force?

• Chemical gradient (Δ in proton concentration)

• Electrical gradient (Δ charges across the membrane)

No PMF → ATP synthase has no energy source → no ATP

What does PMF drive?

• Nutrient transport

• ATP synthesis

• Flagellar rotation

• Reverse electron transport

Why do sulfate reducers grow slower than denitrifiers?

SO₄²⁻ has lower redox potential → less energy yield.

What defines chemolithotrophs?

Use inorganic electron donors (H₂, NH₃, Fe²⁺, H₂S).

They use CO₂ as carbon source → need inorganic energy source.

Why are H₂ oxidizers often microaerophilic?

Hydrogenases are O₂-sensitive.

H₂ → 2H⁺ + 2e⁻

Why is H₂ concentration low in aerobic soils?

Rapid consumption by microbes → must switch to heterotrophy.

Why do sulfur oxidizers store S⁰?

Electron reserve for later oxidation.

Why is sulfur oxidation common at oxic-anoxic interfaces?

equires both:

• H₂S (reduced sulfur)

• O₂ (acceptor)

Why is Fe²⁺ oxidation energy-poor?

Very small ΔE₀’ → low ATP yield.

Why must iron oxidizers live in acidic environments?

At neutral pH, Fe²⁺ oxidizes abiotically → no energy capture.

Why do iron oxidizers grow slowly?

• Low energy yield

• Must oxidize large amounts of Fe²⁺

What are the two steps of nitrification?

• NH₃ → NO₂⁻ (Nitrosomonas)

• NO₂⁻ → NO₃⁻ (Nitrobacter)

Why do nitrifiers grow slowly?

• Small ΔE₀’

• Need reverse electron transport

• Autotrophic (ATP costly)

Why is reverse electron transport needed and what powers it

Some donors (Fe²⁺, NH₃) cannot reduce NAD⁺ directly and uses PMF to power it

however
It consumes energy → less ATP available.

when does fermentation occur and what is its main purpose?

When there is no external electron acceptor available it regenerates NAD+

Why is fermentation less efficient than respiration?

No ETC → no PMF → only substrate-level phosphorylation.

Difference between homolactic vs heterolactic fermentation?

• Homo: glucose → 2 lactate

• Hetero: glucose → lactate + ethanol + CO₂

what is the significance of denitrification pathway and what is its pathway?

Returns nitrogen to atmosphere.

NO₃⁻ → NO₂⁻ → NO → N₂O → N₂

Why are marine sediments black?

H₂S reacts from sulfate reduction reacts with metals to form metal sulfides.

What is the difference between acetogenesis and methanogenesis?

Both are forms of anaerobic respiration where CO₂ is used as the terminal electron acceptor, but they differ in organisms, products, and metabolism:

• Acetogenesis:

  • Performed by acetogenic bacteria

  • Reaction: 2 CO₂ + 4 H₂ → CH₃COOH (acetate) + 2 H₂O

  • More metabolically flexible → can also use sugars and alcohols

  • Produces acetate, which can be used by other organisms

• Methanogenesis:

  • Performed only by methanogenic archaea

  • Reaction: CO₂ + 4 H₂ → CH₄ + 2 H₂O

  • Also use substrates like acetate or methanol

  • Produces methane (CH₄) as end produc

Why do methanogens compete with acetogens in anaerobic environments?

Both groups rely on the same key substrates: hydrogen (H₂) and carbon dioxide (CO₂) as electron donor and acceptor.

• They occupy the same ecological niche

• Availability of H₂ is often limiting, especially in environments like sediments or the gut

• The organism that can use H₂ more efficiently (often methanogens) will outcompete the other

• Methanogenesis is often more energetically favorable, so methanogens can dominate when conditions allow

• Acetogens may dominate when H₂ concentrations are higher or methanogens are inhibited

What is anammox and why is it important?

Anammox (anaerobic ammonium oxidation) is a process where:

NH₃ (or NH₄⁺) + NO₂⁻ → N₂ + H₂O

• Occurs under anaerobic conditions

• Performed by specialized bacteria (e.g., Brocadia)

• Produces nitrogen gas (N₂), removing fixed nitrogen from ecosystems

• Major part of the global nitrogen cycle

• Widely used in wastewater treatment (saves oxygen and energy)

Why is nitrite (NO₂⁻) unique in the anammox process?

• It acts as an electron acceptor when oxidizing ammonia (NH₃ → N₂)

• At the same time, it can act as an electron donor in reactions linked to CO₂ fixation